Cut Phone Line Alarm

Burglars, rapists and other criminals will often cut a phone line before they break into a home. The cut phone line will prevent many alarm systems from alerting the police. The battery powered circuit below will sound an alarm when the unit no longer detects the voltage present on an active phone line.
A bridge rectifier front end makes the circuit phone line polarity independent. It shows four 1N4004 diodes being used but you can also use a standard low current bridge rectifier package with a 200v rating or more. The bridge is wired so the output voltage goes negative relative to circuit ground. The phone line will typically be somewhere around 30v when in standby mode and drop to something less than 15v when a phone is connected to the line. In the circuit, this translates a negative voltage fed to the gate of the 2N7000 FET, which kept the transistor turned off. When the phone line is cut, the voltage fed to the gate of the FET will swing to +9v, which will turn on the FET. When Q1 turns on, it also turns on Q2, which routes power to a 555 timer circuit. The 555 timer is wired as a 1Hz square wave oscillator, which drives the transistor Q3. That transistor drives an alarm beeper. With the components shown, the circuit draws nearly zero current in standby mode. Therefore a good quality 9v battery should last for many years.

Chain Link Fence Alarm

Long stretches of steel fence can be monitored using the simple circuit below. An insulated wire is woven through the links in the fence. Where the fence begins, the circuit is connected to an earth ground and the security wire. At the end of the fence, the wire is connected to one of the steel posts, held in the ground. The typical earth ground resistance will be in the 1000s of ohms range, which is low enough to keep the n-channel FET turned off in the circuit below.

If the wire is cut or broken, the earth ground circuit path is interrupted, allowing the transistor to turn on. This will activate a very loud piezoelectric type siren. I show the use of a cheap 9v battery but a long lasting 12v battery can also be used. The circuit’s current drain is a very low 2 microamps during normal operation, so any battery will last many years. The popular 2N7000 MOSFET transistor is good for about 200ma. I would suggest an n-channel device with a higher current rating, if you plan on using a louder, more powerful siren alarm. As an example, the IRFZ34 device could drive a 10 amp siren. All of these parts are available from www.mouser.com.